1. Field of the Invention
The present invention relates to a scanning projection exposure apparatus, and more particularly, to a scanning projection exposure apparatus for transferring the pattern of a mask onto a photosensitive substrate via a projection optical system while synchronously moving a mask stage for holding the mask, and a substrate stage for holding the photosensitive substrate in a predetermined scan direction.
2. Description of the Related Art
A scanning projection exposure apparatus has been known as one of exposure apparatuses for use in producing semiconductor devices or liquid crystal display devices by the photolithographic process. The scanning projection exposure apparatus, if briefly described, transfers the pattern of a reticle, as a mask, onto a photosensitive substrate via a projection optical system while synchronously scanning a reticle stage (mask stage) for holding the reticle and a substrate stage for holding the photosensitive substrate in a predetermined scan direction.
In a conventional scanning projection exposure apparatus, the reticle stage and the substrate stage were controlled by stage speed control systems assigned respective target values so that the reticle stage and the substrate stage would be synchronously scanned at a speed ratio (e.g., 4:1) determined by the projection magnification ratio of a projection optical system. FIG. 3 shows an example of the speed versus time profile of the substrate stage during scan exposure. As shown in this drawing, the substrate stage is accelerated and reaches a target speed (0.08 m/s, herein). Then, when a predetermined stabilization time (the period of time required for the substrate stage to be synchronized with the reticle stage) has elapsed, an exposure light source is turned on for a predetermined period of time (scan exposure time) to illuminate the reticle with a constant amount of exposure light for performing exposure. After exposure is completed, a predetermined idle time is provided before the stage is decelerated from the target speed to a standstill. The target speed of the stage is determined by the sensitivity of a resist coated onto the surface of the photosensitive substrate so as to give the necessary amount of exposure. That is, the amount of exposure has been determined by the power of the exposure light source (a constant value) multiplied by the speed of the stage (a constant speed).
With this background, development of a high speed stage system is under way to shorten the exposure time and increase the throughput.
To realize such a high speed stage system, the capacity of a power amplifier should be increased to obtain a greater thrust. However, this leads to cost buildup. A laser interferometer, usually used to measure the position of the stage, should be increased in measurement accuracy with the speeding of the stage. As the moving speed of the stage rises, vibrations of the apparatus become large, adversely affecting imaging characteristics. Thus, it is not an easy task to improve the throughput by speeding up the stage.